The present invention relates to a ball for ball games such as a soccer ball, which contains compressed air sealed therein, hits a player's body directly or is kicked with his leg, and so forth. The present invention also relates to a mold for forming a base body of such a ball.
With a ball of this type, since it directly hits a player's body, the hitting impact or shock needs to be mitigated. In addition the ball needs to have an appropriate impact resilience for ease in controlling it with a player's body or leg. Such characteristics are exhibited by the base body of the ball. The base body of the most conventional type comprises a tube of butyl rubber for containing and sealing compressed air and a thread wound reinforcement layer formed by winding the surface of the tube with a thread of nylon or the like having a length of about 3000 m along endless track. Such a base body is bonded at its surface with a plurality of leather panels, with intervention of a thin rubber layer. In the case of a soccer ball, the whole base body thereof is covered with twelve pentagonal leather panels and twenty hexagonal leather panels.
The base body of the above arrangement is formed with many production steps, taking a long time. To overcome such disadvantage, there has been proposed a method for integrally forming a base body of a ball by rotational molding of a thermoplastic elastomer (Japanese Unexamined Patent Publication No. 4634/1978).
In the art disclosed in the above Publication a thermoplastic elastomer, or specifically a polyester elastomer is molded or formed into a base body of a ball. A commercially-available elastomer usually has an extremely wide range of 10% tensile stress from 1 to 200 kg/cm.sup.2. It is extremely difficult to fabricate a ball for practical use without limiting such stress of the base body thereof. This is because insufficient tensile stress would cause the ball to entirely expand with elapse of time due to the internal pressure of compressed air. In contrast, too much tensile stress would cause little deformation of the ball upon impact and hence bring a pain to a player when hit with the ball. In addition a tensile stress out of a pretermined range results in an undesired impact resilience and hence improper bounce.
With regard to molding of a ball, there is known an art for forming a ping-pong ball wherein two hemispherical hollow bodies are formed by injection molding and then bonded together to form a spherical ball, as disclosed, for example, in Japanese Examined Patent Publication No. 41297/1986. As well, there is known another art wherein two hemispherical hollow bodies are formed as a surface layer of a base body and the base body is capped with the two hemispherical hollow bodies, as disclosed, for example, in Japanese Examined Patent Publication No. 26058/1974.
Further, the present inventors have formerly proposed an arrangement wherein a base body is formed of a spherical hollow body which is composed of two hemispherical hollow bodies obtained by injection-molding a thermoplastic elastomer. The two hollow bodies are bonded together. This arrangement is disclosed in Japanese Unexamined Utility Model Publication No. 88567/1991. In this publication, one hemispherical hollow body is molded integrally with a valve at the center thereof. FIG. 12 of the instant application illustrates such a mold for forming such valve-integrated hemispherical hollow body. Denoted by numerals 51 and 52 are an upper mold and a lower mold, respectively, between which a cavity 53 for forming one hemispherical hollow body is defined. A core 54 is provided for forming a valve and integrally formed with the upper mold 51. A runner 55 for injection molding is extending centrally through the core 54. Denoted by numeral 56 is a valve-forming cavity, by numeral 57 a recessed portion defined in the lower mold 52 at a position corresponding to the exit of the runner 55 for facilitating the flow of molten thermoplastic elastomer from the runner 55 to the cavity 56.
The mold structure shown in FIG. 12 is developed for ease of releasing a molded hemispherical hollow body from the upper mold 51 having the core 54. However, there still remains troublesome work for the operator, for example, to withdraw the molded hemispherical hollow body to release it from the upper mold 51.
A large-diameter portion of the core 54 needs to be relatively large for maintaining a valve rubber, but if it is too large, the hemispherical hollow body is hard to be released. Accordingly, the diameter of the large-diameter portion is restricted in view of the difficulty of releasing. This results in weakened force for maintaining the valve rubber and insufficient air-sealing.
There is another problem that a mass of resin formed in the recessed portion 57 needs to be removed by the operator after the molding.
The present invention has been achieved in view of the above-mentioned circumstances. More specifically, the present inventors have found, as a result of various experiments on the requirements for a ball, wherein base bodies were formed using various thermoplastic elastomers, the fact that if the tensile stress to which the inventors' attention is directed is set within a given range, various properties of a ball such as shock-absorptivity, shape-maintaining property, impact resilience and the like can be set within appropriate ranges. Thus, a first invention of the present invention has been achieved.
On the other hand, a second invention of the present invention is designed so that after molding of a hemispherical hollow body, the body can be automatically released without leaving an unnecessary mass of resin on the body.